Section 11: Spatial Relationships Exercises¶

Here’s a reminder of the functions we saw in the last section. They should be useful for the exercises!

sum(expression) aggregate to return a sum for a set of records
count(expression) aggregate to return the size of a set of records
ST_Contains(geometry A, geometry B) returns true if geometry A contains geometry B
ST_Crosses(geometry A, geometry B) returns true if geometry A crosses geometry B
ST_Disjoint(geometry A , geometry B) returns true if the geometries do not “spatially intersect”
ST_Distance(geometry A, geometry B) returns the minimum distance between geometry A and geometry B
ST_DWithin(geometry A, geometry B, radius) returns true if geometry A is radius distance or less from geometry B
ST_Equals(geometry A, geometry B) returns true if geometry A is the same as geometry B
ST_Intersects(geometry A, geometry B) returns true if geometry A intersects geometry B
ST_Overlaps(geometry A, geometry B) returns true if geometry A and geometry B share space, but are not completely contained by each other.
ST_Touches(geometry A, geometry B) returns true if the boundary of geometry A touches geometry B
ST_Within(geometry A, geometry B) returns true if geometry A is within geometry B

Also remember the tables we have available:

nyc_census_blocks
- blkid, popn_total, boroname, geom
nyc_streets
- name, type, geom
nyc_subway_stations
- name, geom
nyc_neighborhoods
- name, boroname, geom

Exercises¶

“What is the geometry value for the street named ‘Atlantic Commons’?”

SELECT ST_AsText(geom)
  FROM nyc_streets
  WHERE name = 'Atlantic Commons';

MULTILINESTRING((586782 4504202,586864 4504216))

“What neighborhood and borough is Atlantic Commons in?”
```
SELECT name, boroname
FROM nyc_neighborhoods
WHERE ST_Intersects(
  geom,
  ST_GeomFromText('LINESTRING(586782 4504202,586864 4504216)', 26918)
);
```
```
    name    | boroname
------------+----------
 Fort Green | Brooklyn
```
Note

“Hey, why did you change from a ‘MULTILINESTRING’ to a ‘LINESTRING’?” Spatially they describe the same shape, so going from a single-item multi-geometry to a singleton saves a few keystrokes.

More importantly, we also rounded the coordinates to make them easier to read, which does actually change results: we couldn’t use the ST_Touches() predicate to find out which roads join Atlantic Commons, because the coordinates are not exactly the same anymore.

“What streets does Atlantic Commons join with?”

SELECT name
FROM nyc_streets
WHERE ST_DWithin(
  geom,
  ST_GeomFromText('LINESTRING(586782 4504202,586864 4504216)', 26918),
  0.1
);

     name
---------------
 S Oxford St
 Cumberland St

“Approximately how many people live on (within 50 meters of) Atlantic Commons?”

SELECT Sum(popn_total)
  FROM nyc_census_blocks
  WHERE ST_DWithin(
   geom,
   ST_GeomFromText('LINESTRING(586782 4504202,586864 4504216)', 26918),
   50
  );

Previous: Section 10: Spatial Relationships

Next: Section 12: Spatial Joins

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Section 11: Spatial Relationships Exercises
- Exercises

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Previous: Section 10: Spatial Relationships

Next: Section 12: Spatial Joins

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Introduction to PostGIS